1. Field of the Invention
This invention relates to a chemically sensitive field effect transistor, particularly an ion-sensitive field-effect transistor.
2. Description of the Prior Art
A known chemically sensitive "field-effect transducer", (for terminology see: ETZ-A, Vol. 16 (1964), No. 23, P. 682), of this kind (German Offenlegungsschrift No. 26 10 530, and also Biomedical Engineering, July 1976, pages 241 to 245) does not have a metal gate electrode integrated in the semiconductor body, unlike a likewise known device specially intended for detecting hydrogen (German Offenlegungsschrift No. 25 40 161). On the contrary, its chemically sensitive and selective layer is brought into contact with the material being examined by means of a preferably aqueous solution in which a reference electrode is immersed. The sensitive layer interacts with determined substances in the aqueous solution, so that in this way an electric field produced between source and drain in the semiconductor material is modulated in accordance with the chemical properties of the substance. These variations can be detected by an amplifier such as is known for example from IEEE Transactions on Biomedical Engineering, Vol. BME-19, No. 5, Sept. 1972, pages 342 to 351.
The industrial application of the field-effect transducer known from German Offenlegungsschrift No. 26 10 530 entails various problems. Apart from the fundamental problem of transferring the technique of chemically sensitive electrodes to semiconductor components, there is an additional technological problem. The chemically sensitive layer of the chemically sensitive transducer must be in direct contact with the conductive solution, which as previously mentioned is usually an aqueous solution, for example in the form of a pH-sensitive element, without any conductive connection being made between the transducer and the solution. In the planar technology customary at the present time, however, all electrical supply leads are disposed on a single surface of a semiconductor component of this kind, and in the known case the chemically sensitive layer is also situated on that surface. This gives rise to difficult insulation problems.